Controlled electron multiplier



April 3, 1951 J. c. FERGUSON CONTROLLED ELECTRON MULTIPLIER Filed Oct. 24, 1946 FIG! SIGNA L SOU RC E OUTPUT 22 y I4 ,45 IS I? I8 AAAAAA llllAAl Alllll ll '77 M ll OUTPUT CURRENT I/OUTPUT CURRENT I TIME INPUT VOLTAGE FIG.2

INVENTOR JOSEPH C. FERGUSON IN PUT VOLTAGE ATTORNEY Patented Apr. 3, 1951 UNITED STATES PATENT OFFICE CONTROLLED ELECTRON MULTIPLIER Application October 24, 1946, Serial No. 705,413

6 Claims.

This invention relates to electron multipliers and particularly to a means for and method of controlling a multiplier so that the steady component of the electron current flowing through the multiplier is minimized.

Electron multipliers are known to be eiiicient amplifiers of signal in a very wide frequency range. However, an input signal such, for example, as a modulated carrier wave or an audio signal has an average level that may vary appreciably. If the electron multiplier is operated at a constant mean current level, an input signal having, for example, an instantaneous low signal level will vbe amplified with a steady current component which may become very large in view of the high amplification factor of an electron multiplier. This steady current component of the output signal represents a waste of power, decreases the efiiciency of the multiplier and reduces the useful life thereof because the multiplier is needlessly called upon to transmit large currents. It is desirable, therefore, to operate an electron multiplier in such a manner that the steady current component associated with the signal current flowing through the multiplier is minimized, thereby to improve the eiiiciency thereof.

It has previously been suggested to apply automatic volume control to an electron multiplier. The purpose of an automatic volume control is to keep the mean amplitude or the peak amplitude of the carrier wave constant in spite of variations thereof which may be caused by fading. The automatic volume control is usually applied to the radio-frequency and to the intermediate-frequency amplifier stages of a superheterodyne receiver. It has furthermore been proposed to regulate a multiplier in such a manner that its gain is kept constant. Thus, when a multiplier is provided with a photoelectric cathode which may be illuminated by a light source, the gain of the multiplier may be kept constant in spite of variations of the intensity of the light source. In all these multiplier circuits the steady electron current component is increased rather than decreased.

It is an object of the present invention, therefore, to provide an electron multiplier wherein the steady current component is minimized, thereby to improve the efiiciency of the multiplier.

A further object of the invention is to provide an electron multiplier wherein the intensity of the modulated electron stream passing through the multiplier is controlled in direct proportion to the average level of the input signal.

In accordance with the invention there is provided an electron multiplier comprising means for developing a primary electron beam and means for modulating the intensity of the primary electron beam in accordance with an input signal. There is further provided a secondary electron emissive stage arranged to receive and multiply the primary electron beam and a collector electrode is arranged to receive the electrons liberated from the multiplying stage. Finally there are provided means for deriving a control signal representative of the average value of the input signal and means for controlling the intensity of the primary electron beam in accordance with the control signal in direct proportion to the average value of the input signal. Thus, the steady component of the electron current flowing through the multiplier is minimized which will, in turn, improve the efficiency of the multiplier.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a circuit diagram of an electron multiplier including a control circuit embodying the present invention, while Fig. 2 is a graph illustrating curves representing the input voltage and the output current of input signals having different average levels.

Referring to Fig. 1, there is illustrated electron multiplier l comprising cathode 2, control grid 3, a plurality of secondary electron emissive stages such as 4, 5, 6 and 1 and collector electrode 8 all arranged in evacuated envelope It. For the purpose of supplying operating potentials to the electrodes of multiplier I, there may be provided a suitable voltage source such, for example, as battery if having its terminals connected across voltage divider 22. The negative terminal of battery I! may be grounded as shown.

By means of tap E3 on voltage divider l2 cathode 2 may be kept at a suitable potential which may be positive with respect to ground. Multiplying stages 4 to l are connected, respec tively to taps i4, I5, [6 and IT for maintaining them at increasing positive potentials with respect to cathode 2. Collector electrode 8 is supplied with a potential that is positive with respect to that of last multiplying stage 1 by connecting it to tap l8 through load impedance as which may be an inductance as illustrated. The output signal may be obtained from output terminals 2| inductively coupled to load inductance 2|] through coil 22.

Control grid 3 is normally maintained at a potential that is negative with respect to that of cathode 2 through tap 24 on voltage divider E2. The input signal is developed by signal source 25 and impressed upon control grid 3 through coupling condenser 25. The signal developed by signal source 25 may consist, for example, of a modulated carrier wave or of an audio or video signal.

The electron multiplier as described hereinabove operates in a conventional manner. The primar electron beam developed by cathode 2 has its intensit modulated by control grid 3 in accordance with the input signal derived from signal source 25. Multiplying stages 4 to 1 are arranged to receive and multiply the primary modulated electron beam which is collected by collector electrode 8. The amplified output signal developed across load inductance 20 may be obtained from output terminals 2|.

Referring now to Fig. 2, there is shown curve 30 which indicates schematically the relationship between the input voltage impressed upon control grid 3 and the output current flowing through load inductance 20. Below curve 30 the input voltage of two different input signals 3| and 32 has been plotted against time, while to the right of curve 30 output current waves 33 and 34 corresponding to input voltage waves 3| and 32 have been plotted against time. Let it be supposed that input voltage wave 3| is impressed upon control grid 3. The output current wave will then have the form illustrated at 33. If, however, the input voltage has the form shown at 32, the grid bias must be increased to a larger value shown at 35 so that the output current wave 34 will not be distorted. On the other hand, if input voltage wave 3| is amplified, with a fixed large grid bias such as indicated at 35, it will be evident that an appreciable steady component of the electron current will flow through multiplier I. It has already been pointed out that this steady current component represents a loss of power and a decrease in eificiency.

In. accordance with the present invention the bias of control grid 3 is controlled in direct proportion to the average value or mean level of the input signal. For this purpose there may be provided control circuit 33 for deriving a control signal representative of the average level of the input signal. Control circuit 38 comprises amplifier 49 upon the grid of which the input signal developed by signal source 25 may be impressed. For the purpose of providing self-bias cathode resistor 4| may be bypassed by condenser 42 and connected between the cathode of amplifier 4|! and ground. The anode of amplifier 46 may be connected to tap 43 on voltage divider I2 through a suitable anode impedance such as tuned circuit 44. In case the input signal developed by signal source 25 is not a modulated carrier wave, tuned circuit 44 may be replaced by a non-resonant impedance such as a resistor.

The amplified input signal developed across tuned circuit 44 may be impressed on anode 45 of rectifier or diode 46. Anode 45 of diode 45 may be connected to tap 24 on voltage divider |2 through resistor 47. Cathode 48 of diode 45 is also connected to tap 24 through resistor 50 which may be bypassed by condenser for pro viding an alternating current bypass.

Control circuit 38 operates as follows. Let it be assumed that the average level of the input signal developed by signal source 25 increases. Thus, the input signal may change from voltage wave 3| to voltage wave 32. Voltage wave 32 is impressed on the control grid of amplifier 40 so that a larger alternating current will fiow through tube 46 and tuned circuit 44. Accordingly, the voltage impressed on anode 45 of diode 46 is reduced so that less rectified current will flow through diode 46 and the parallel combination of resistor 50 and condenser 5|. The bias voltage applied to control grid 3 of multiplier will accordingl rise. On the other hand, if the average value of the input signal decreases, less alternating current will flow through amplifier 40 and tuned circuit 44. Consequently the voltage impressed upon anode 45 of diode 46 increases so that more rectified current will fiow through diode 45 and resistor 50 and condenser 5|.

Preferably the time constant of resistor 5|] and condenser 5| is such that rapid changes of the average input signal level are not impressed upon control grid 3 but only slow variations thereof. Thus, if for example the input signal developed by signal source 25 is an audio signal, the time constant of resistor 50 and condenser 5| may be of the order of one-tenth of a second. It will therefore be seen that the intensity of the primary electron beam in multiplier is controlled directly in proportion with the average Value of the input signal with a resulting increase of the efiiciency of the multiplier. The steady current component of the electron current flowing through multiplier i is thus minimized in accordance with the invention.

It will be understood that the control signal which is developed by control circuit 38 may be obtained in any suitable manner. Thus, the control signal may be derived from the output voltage of a previous intermediate-frequency or radio-frequency amplifier or multiplier. It is also feasible to derive the control signal from the output circuit of electron multiplier Amplificati-on of the control signal is not essential for the operation of the present invention and may be omitted when the signal from which the control signal is derived is of sufficient amplitude to vary the bias of control grid 3.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron multiplier comprising means for developing a primary electron beam, means for modulating the intensity of said primary beam, a secondary electron emissive stage arranged to receive and multiply said primary beam, a collector electrode arranged to receive the electrons liberated from said stage, a source of input signal connected to said modulating means, means for deriving a control signal representative of the average value of said input signal, and means for controlling the steady current component of said primary beam in accordance with said control signal in direct proportion to the average value of said input signal comprising means for connecting said deriving means to said modulating means, thereby to improve the efiiciency of said multiplier.

2. An electron multiplier comprising means for developing a primary electron beam, means for modulating the intensity of said primary beam, a plurality of secondary electron emissive stages arranged to receive and multiply said primary beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, a source of input signal connected to said modulating means, means for deriving a control signal representative of the average level of said input signal, and means for controlling the steady current component of said primary beam in accordance with said control signal in direct proportion to the average input signal level comprising means for connecting said deriving means to said modulating means, thereby to reduce the steady current component flowing through said multiplier.

3. An electron multiplier comprising means for developing a primary electron beam, a control grid for modulating the intensity of said primary beam in accordance with an input signal, a plurality of secondary electron emissive stages arranged to receive and multiply said primary beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, means for deriving a control signal representative of the average value of said input signal and means for impressing said control signal on said grid thereby to control the steady current component of said primary beam in direct proportion to the average value of said input signal and to improve the efliciency of said multiplier.

4. An electron multiplier comprising a source of primary electrons, a control grid for controlling the intensity of the primary electron stream developed by said source, a secondary electron emissive stage for multiplying said primary electrons, a collector electrode for collecting the multiplied electrons, a signal source coupled to said grid, means for deriving a control signal representative of the average signal level and means for impressing said control signal upon said grid so that'the steady current component of said primary electron stream varies in direct propor- 6 tion to said average signal level, thereby to improve the efficiency of said multiplier.

5. An electron multiplier comprising a source of primary electrons, a control grid for controlling the intensity of the primary electron stream developed by said source, a plurality of secondary electron emissive stages for multiplying said primary electrons, a collector electrode for collectin the multiplied electrons, a signal source coupled to said grid, means for de ing a control signal representative of the average signal level, and means for impressing aid control signal upon said grid in such a polar; 1 that the steady current component of said primary electron stream increases when said average signal level increases, thereby to reduce the steady current component flowing through said multiplier.

5. An electron multiplier comprising a source of primary electrons, a control grid for controlling the intensity of the primary electron stream developed by said source, a plurality of secondary electron emissive stages for multiplying said primary electrons, a collector electrode for collecting the multiplied electrons, a signal source coupled to said grid, means for deriving a control signal representative of the average signal level, means for amplifying said control signal, and means for impressing said control signal upon said grid in such a polarity that the steady current component of said primary electron stream varies in the same direction as said average signal level, thereby to improve the efficiency of said multiplier.

JOSEPH C. FERGUSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,200,037 Mountjoy et al May '7, 1940 2,298,960 McRae Oct. 13, 1942 2,412,423 Rajchman et a1. Dec. 10, 1946 

